Various nanofabrication, lithography, sample preparation, metrology, or inspection applications of focused ion beam (FIB) systems require very accurate placement of the FIB milling beam (on a nanometer scale). When the FIB beam is employed to form an image of the work piece to align the beam for milling, sample damage may result by exposing sensitive regions to the high-energy ion beam.
Dual beam systems, including a FIB and a scanning electron microscope (SEM), have been introduced which can image the sample with the SEM and mill on the sample using the FIB. Some dual beam instruments utilize coincident FIB and SEM beams, where the beams are incident upon the surface with a large angle between them. The sample may be tilted towards or away from each beam axis to facilitate various SEM and FIB milling operations. Another type of dual beam instrument employs parallel FIB and SEM beams or adjacent beams with an angle between the beams with sample stage motion between the two beams. In either case, the placement accuracy of the FIB beam to the SEM image has limitations due to stage inaccuracies, sample non-flatness, and—in the case of coincident beams—the different beam angles.
What is needed is a way to combine the micromachining capabilities of a FIB system with the superior and non-destructive imaging of a SEM (so that a SEM image could be used to align the FIB for milling) without the placement inaccuracy inherent in existing dual beam designs. A number of steps in the mass production of nanodevices would benefit from such a combination FIB-SEM, including, for example, the manufacture of thin film magnetic heads for use in computer hard drives or similar devices. The phrase “mass production of nanodevices” as used herein encompasses manufacturing, nanofabrication, lithography, sample preparation, metrology, and inspection applications.
As the computer industry continues to demand higher capacity and faster performance from hard disks and tape drives, there is an increasing demand for suppliers to increase the amount of data that can be stored on a given storage medium. This amount of data, referred to as areal density, is usually expressed as the number of bits of data per square inch of storage media.
In a typical hard disk, the data is stored on round, flat disks called platters, usually made of glass or an aluminum alloy. A platter is coated on both sides with a very thin layer of magnetic material, which is designed to store information in the form of magnetic patterns. The platters are mounted inside the hard disk cover by cutting a hole in the center of each platter and stacking several platters onto a spindle. The platters can be rotated at high speed by a motor connected to the spindle. Special electromagnetic read/write devices called heads are used to either record information onto the disk or read information from it. These heads are mounted onto sliders, which are in turn mounted onto arms. All of the arms are mechanically connected into a single assembly and positioned over the surface of the disk by a device called an actuator. In this fashion, the read/write heads can be accurately positioned over the surface of the platters.
The read/write heads transform electrical signals to magnetic signals, and magnetic signals back to electrical ones again. Each bit of data to be stored is recorded onto the hard disk using a special encoding method that translates zeros and ones into patterns of magnetic flux reversals.
Each surface of each platter on the disk can hold tens of billions of individual bits of data. These are organized into larger “chunks” for convenience, and to allow for easier and faster access to information. Each platter has two heads, one on the top of the platter and one on the bottom, so a hard disk with three platters would normally have six surfaces and six total heads. Each platter has its information recorded in concentric circles called tracks. Each track is further broken down into smaller pieces called sectors, each of which holds 512 bytes of information.
The portion of a write head that actually writes data on the disk is referred to as the write element. This element is typically made up of two poles that are separated by a gap. These poles generate a magnetic field when they are excited by a coil magnetically coupled to the poles. When the write element is in proximity to the disk, a magnetic field generated by the poles sets the magnetic orientation in given locations on the disk. In this manner, data is written on the disk.
One of the major factors that determines the areal density of a hard disk is the track density. This is a measure of how tightly the concentric tracks on the disk can be packed.
Track density is largely determined by the size of the area of the disk that is affected by the write head. A large head structure will affect a larger area on the surface of a platter than will a smaller head structure. As a result, track width can be decreased (and track density increased) by making the poles of the write head physically smaller at the write tip, thereby concentrating the magnetic field into a smaller area on the platter surface.
A large percentage of the write heads used today are thin-film heads, so named because of the way in which they are manufactured. Thin film heads are made using a photolithographic process similar to the way integrated circuits are made. During the manufacturing process, a substrate wafer is coated with one or more layers of a very thin film of alloy material deposited in specific patterns. Alternating layers of an insulating material are also deposited onto the substrate. Lithographic techniques are used to form the deposited layers into a pole-tip assembly having the desired geometry.
However, there is a limit on how small a write head can be manufactured using lithographic techniques alone. Smaller write heads often require micromachining with a focused ion beam device.
FIB systems are widely used in microscopic-scale manufacturing operations because of their ability to image, etch, mill, deposit, and analyze very small features with great precision. Ion columns on FIB systems using gallium liquid metal ion sources (LMIS), for example, can provide five to seven nanometer lateral imaging resolution. Focused ion beams mill by sputtering, that is, physically removing atoms and molecules from the specimen surface. Because of their versatility and precision, FIB systems are employed in the fabrication of thin film magnetic heads used for writing information to data storage media.
FIB systems operate by directing a focused beam of ions over the surface of a work piece, typically in a raster pattern. The ions are typically extracted from a liquid metal ion source (LMIS). The extracted ions are accelerated, collimated, and focused onto a work piece by a series of apertures and electrostatic lenses. Electrostatic lenses are used because the ions are too massive to be focused by reasonably sized magnetic lenses; gallium ions are about 128,000 times heavier than electrons. A common type of electrostatic lens used to focus an ion beam is an einzel lens. An einzel lens is a unipotential lens with three electrodes or elements. Typically, the center element is at a high positive potential and the upper and lower elements are maintained at ground potential.
When a FIB system is used, first the ion beam typically scans the surface of the specimen in a raster pattern and secondary electrons are collected to form an image of the specimen surface. This image can be used to identify the features to be milled. The ion beam scan pattern is then adjusted to coincide with the feature to be milled, and the ion beam is used to mill the surface. A gaseous material is often directed to the work piece at the impact point of the ion beam, and the ions induce a chemical reaction that selectively either increases the etch rate or deposits material, depending on the gaseous compound that is used. Unfortunately, thin film head trimming and other nanofabrication applications may suffer from sample damage during the FIB image processes. If the FIB beam image time is reduced to decrease sample damage, the image signal-to-noise ratio may be insufficient for accurate milling placement. In other cases, it may be undesirable to ever direct the FIB beam at certain sensitive structures.
In contrast to FIB imaging, a low energy electron beam as used in a scanning electron microscope (SEM) causes less damage to the work piece and has greater alignment accuracy. In a scanning electron microscope, a finely focused beam of electrons is scanned across the surface of a work piece. The electron beam originates from an electron source and the electrons are accelerated toward the work piece by a voltage, usually between 0.2 kV and 30 kV. That beam is typically collimated by electromagnetic condenser lenses, focused by an objective lens, and scanned across the surface of the work piece by electromagnetic deflection coils. When the electrons in the electron beam strike the work piece surface, secondary electrons are emitted. As in a FIB system, these secondary electrons are collected and used to form an image of the work piece surface in which the brightness of each point on the image is determined by the number of secondary electrons ejected while the primary electron beam was impinging at that point. The finely focused electron beam of an SEM allows for the production of an image of greater magnification and higher resolution than can be achieved by the best optical microscopes.
Since electrons can be focused either by electrostatic forces or magnetic forces, both electrostatic and magnetic lenses can be found in SEMs. Electrostatic lenses usually have larger aberrations than magnetic lenses (particularly for higher beam voltages) so they are not as commonly used.
Unlike imaging with an ion beam, SEM imaging usually does not significantly damage a work piece surface. This is because electrons cannot sputter material the way that ions can. The amount of momentum that is transferred during a collision between an impinging particle and a substrate particle depends not only upon the momentum of the impinging particle, but also upon the relative masses of the two particles. Maximum momentum is transferred when the two particles have the same mass. When there is a mismatch between the mass of the impinging particle and that of the substrate particle, less of the momentum of the impinging particle is transferred to the substrate particle. A gallium ion used in focused ion beam milling has a mass of over 128,000 times that of an electron. As a result, the momentum of particles in a gallium ion beam is sufficient to sputter molecules from the surface. However, the momentum of an electron in a typical SEM electron beam is not sufficient to remove molecules from a surface by momentum transfer.
Although an SEM beam is typically much less destructive than an ion beam, certain work pieces, such as some integrated circuits, are susceptible to damage by higher energy electron beams. For this reason, flexibility in electron landing energies, particularly in relatively low voltage ranges, is important when using the electron beam to view these types of work pieces.
The Dual FIB-SEM systems currently available use separate optical columns for the ion and electron beams, and typically there is about 52 degrees between the two beam axes. Minor differences in the distances between final lens of each system and the work piece affect the relative positions of the two beams, so there is always misalignment when switching between the beams. Because there are two different optical columns (each at a different angle to the sample), the sample sometimes has to be tilted to change from FIB to SEM operation (or from SEM to FIB). Some sample displacement is inevitably caused during stage tilt. During any subsequent process such as high precision micromachining, the beam placement accuracy is decreased. Even if the sample displacement problem is overcome, the images obtained by the electron beam and the ion beam will still be different since the beams are incident from two directions. In addition, if the detector is on the side, image shadowing will result, which is sometimes undesirable.
For a combined FIB-SEM to be optimal for the mass production of nanodevices, such as thin film magnetic head manufacturing, both beams should preferably come from above the work piece in a coaxial fashion, so that minor variations in work piece distance from the final lens will not affect the relative positions of the two beams on the same work piece. Because of the high degree of accuracy required, any displacement of either the work piece and particle beam sources must be avoided.
While coaxial column FIB-SEM systems have been described in the literature, all such systems existing to date suffer from design characteristics that limit their usefulness for high-precision or mass production applications.
A single optical column FIB and electron beam system is described in U.S. Pat. No. 4,740,698, to Tamura et al. for “Hybrid Charged Particle Apparatus.” In this system, however, separate ion and electron sources are mounted on a changeover device employed to switch between ion and electron beams. The displacement of source position involved in the switch-over also leads to a relative shift between the FIB and the electron beam images.
Another single column FIB/electron beam device is described in Japanese Patent No. 63-236251, to Sawaragi for “Electron Beam/Ion Beam Combination Device.” In this system, however, an electromagnetic lens is used to focus the ion beam while a separate electrostatic lens is used to focus the ion beam. When the electron beam is in use, the electromagnetic lens used to focus the electron beam is switched on, but the electrostatic lens is switched off. Before the ion beam can be used, the electron beam is shut off and the electromagnetic lens is switched off. The ion beam and the electrostatic lens are then switched on. As a result, the beams cannot operate simultaneously. When a lens is switched on, a period of time is required for the lens to stabilize, making such a system unsuitable for a high throughput production environment. Also, magnetic lenses have hysteresis effects, which inhibit accurate beam placement after shutting the lens off.
Another single column FIB/electron beam device is described in Japanese Patent No. 02-121252, to Sawaragi for a “Charged Particle Beam Combination Device.” (“Sawaragi II”). This system uses a combination of a magnetic and an electrostatic lens in series to focus the electron and ion beams respectively. Although this system does allow simultaneous use of the beams, the use of lenses in series results in an increase in focal length, which typically degrades the resolution of the FIB system. The Sawaragi II patent also uses post-lens deflection to position the beams. This also requires an increased focal length and limits secondary electron collection efficiency and accuracy when using through-the-lens detection of secondary electrons (discussed in greater detail below). As a result, the design of the Sawaragi II patent does not provide the resolution required for advanced thin film head trimming and other nanofabrication applications. Further, a combination lens system will necessarily be physically larger than a single lens system. Smaller systems are desirable because many systems are located in a clean room, and the cost of clean room space is extremely high. Finally, the combination lens system used by the Sawaragi II patent is more complex than a single lens system. This tends to reduce the reliability of such a system and make it less suitable for mass production manufacturing. Still another single column FIB/electron beam device is described in Cleaver et al., “A Combined Electron and Ion Beam Lithography System,” J. Vac. Sci. Technol. B, 144 (1985) (hereinafter “Cleaver”). However, the system described in Cleaver uses an einzel lens—a unipotential electrostatic lens—to focus both the ion and electron beams. As such, the ratio of the ion beam energy to the electron beam energy determines the distance from the final lens at which the beams focus. Changing the energy of either beam therefore requires that the work piece be moved to be at the new focus point for that beam. The Cleaver system does not provide the flexibility to readily adjust the beam voltage for different applications. For example, in semiconductor applications, users prefer a low SEM beam voltage to avoid surface damage by electrical charging. Further, any adjustment in beam voltage requires a change in the voltage of the middle element of the einzel lens. A large voltage change in the final lens can take as much as one second—a significant amount of time in the mass production process.
Thus, a single column FIB-SEM system suitable for high-accuracy mass production of nanodevices is still needed.